A new study examines the complex relationship between Atlantic spotted and bottlenose dolphins in the Bahamas, where the two species have forged a fascinating alliance.

Two species of dolphins in the Bahamas have developed an alliance, a new long-term study reports. Researchers have seen Atlantic spotted and bottlenose dolphins playing together, foraging together and teaming up to fend off intruders. They've even seen adults from one species babysitting calves from another.

This isn't the only instance of dolphins interacting across species, but it is the most complex such dynamic known to science. Aside from primates, not many mammals have been closely studied cooperating with other mammal species over time. Dozens of dolphins and whales have been seen in mixed-species groups, yet these sightings are often rare and short-lived, yielding mainly anecdotal descriptions.

The Bahamian bottlenose and spotted dolphins, however, have been studied for the past 30 years by the Florida-based Wild Dolphin Project. And thanks to a new paper published by those researchers in the journal Marine Mammal Science, we now have unprecedented insights into the complex relationship the two species have forged.

"What is unique about our study is that we can actually see them underwater, so we know what behaviors they are actually doing together," study co-author and Wild Dolphin Project founder Denise Herzing tells MNN. "They travel together, socialize together, form interspecific alliances when threatened, babysit each other's calves."

The spotted dolphins seem to spend about 15 percent of their time with bottlenose dolphins, and about two-thirds of those interactions are cooperative. Males from each species have been seen teaming up to chase away an intruder, for example, and maturing female spotted dolphins are known to care for bottlenose calves when in mixed groups. ("So far, not the other way around," Herzing notes, although pregnant females of both species have been documented hanging out together.)

The motivations behind this are still unclear, but Herzing and co-author Cindy Elliser of Pacific Mammal Research say it's too consistent to be a fluke. The two dolphin species seem to be doing the kinds of things humans and other primates do to maintain friendly alliances. And that could give them both an evolutionary edge.

Atlantic spotted dolphins are dark gray as calves, then develop their namesake spots as they mature. (Photo: NOAA)

"These interactions likely evolved to allow the species to share space and resources and maintain a stable community," Elliser tells New Scientist. It also boosts safety, Herzing adds. "Better to know your neighbor when you are in trouble than not."

This level of collaboration offers further evidence of dolphins' complex social lives, as seen in other behaviors like calling one another by name and using diplomacy to defuse fights. Like most relationships, however, even this friendly dynamic includes a mix of camaraderie and combat. While most of the dolphins' interactions are cooperative, about 35 percent are "aggressive," Herzing says.

There's a notable size difference between the two species — bottlenose dolphins can grow up to 12.5 feet long and 1,400 pounds, compared with 7.5 feet and 315 pounds for Atlantic spotted dolphins. Adult male bottlenoses sometimes use their size to harass their smaller associates, reportedly forcing their way into spotted dolphin groups and mating with the females. They've even been seen mounting male spotted dolphins as a dominance display, according to IFLScience.

Spotted dolphins are no pushovers, though. The males are known to repel these attacks by organizing themselves into large, synchronized groups that intimidate their bottlenose bullies. The exact nature of the species' relationship is still murky, but this suggests the spotted dolphins need to exert more effort — both cooperatively and combatively — to make up for their size disadvantage.

The alliance may not be entirely balanced, but it does seem adaptive for both parties. And this kind of behavior could be especially useful, according to Elliser, as climate change drives species out of their habitats and forces them to share space. "These types of interactions in social animals may become more common," she says.